This application claims the priority of German patent document 198 32 389.1, filed Jul. 18, 1998 and PCT International Application No. PCT/EP99/04353, filed Jun. 23, 1999, the disclosures of which is expressly incorporated by reference herein.
The invention relates to a polymer electrolyte membrane fuel cell system.
The generation of a hydrogen-rich gas from a crude fuel for the operation of fuel cells provides a product gas with a carbon monoxide content of a few per cent. During the steam reforming of methanol, the reformate which is formed is at a temperature of, for example, approximately 300xc2x0 C. Before it enters the fuel cell, the carbon monoxide concentration has to be reduced to approximately 10 ppm and the reformate has to be cooled to the operating temperature of the fuel cell, which is usually in the region of approximately 80xc2x0 C. For this purpose, in conventional fuel cell systems, a gas-cleaning stage is provided between the gas-generation device and the fuel cell. Furthermore, reformate coolers are provided between the gas-generation device and the gas cleaning or between the gas-cleaning stage and the fuel cell.
European patent document EP 0 743 694 A1 discloses a fuel cell system in which a hydrogen-rich gas containing carbon monoxide is generated from a methanol/water mixture, with the aid of a reformer. The carbon monoxide is removed from the reformate in a gas-cleaning stage with the aid of selective oxidation, while oxygen is added. To cool the gas-cleaning stage, a heat exchanger is provided, which has water or oil flowing through it. A further liquid-air heat exchanger is provided for cooling the cooling medium.
Furthermore, U.S. Pat. No. 5,271,916 discloses a two-stage device for the selective oxidation of carbon monoxide in a hydrogen-rich gas mixture. This gas-cleaning stage has a heat exchanger, which preferably has a liquid cooling medium with a boiling point of between 160xc2x0 and 175xc2x0 C. flowing through it. Before it enters a downstream fuel cell, the gas mixture is cooled to the necessary fuel cell temperature in a further heat exchanger, which has water flowing through it.
Finally, International patent document WO 93/19005 A1 discloses a generic fuel cell system in which carbon monoxide contained in a hydrogen-rich gas mixture is selectively oxidized in a two-stage gas-cleaning stage while oxygen is being added. Heat exchangers which have a liquid flowing through them are provided in both gas-cleaning stages. In addition, a further heat exchanger is provided between the two stages.
One object of the present invention is to provide a compact fuel cell system which is inexpensive to produce and is improved in terms of the cooling of the gas-cleaning stage.
These and other objects and advantages are achieved by the fuel cell system according to the invention, in which a gas-cleaning stage is designed as a gas-gas heat exchanger that uses the anode off-gas and/or cathode off-gas from the fuel cell as a cooling medium. This configuration provides a simplified device, since there is no need for an additional coolant circuit with associated liquid/air heat exchanger. As a result, the construction space required is reduced and the costs of the device reduced.
Moreover, the inherently lower transfer of heat in gas/gas heat exchangers prevents excessive dissipation to the cooling medium. As a result, it is possible to prevent the reaction from being cooled excessively in the final gas-cleaning stage, which would make it impossible for any further oxidation to take place. (The extent of covering of the catalyst with carbon monoxide would become too high.)
Furthermore, the enthalpy flow on the cooling side (i.e., in the anode off-gas and/or cathode off-gas) is load-dependent, so that according to the heat of reaction developed during the selective oxidation, more energy is dissipated at a high load while less energy is dissipated under a low load. Moreover, the overall efficiency of the system is improved if the anode off-gas and/or cathode off-gas is fed to a downstream catalytic burner, since the anode off-gas and/or cathode off-gas is preheated as it flows through the heat exchanger. This energy then no longer has to be supplied in the catalytic burner. Overall, the cooling of the gas-cleaning stage does not cause any thermal energy to be lost to the overall system.
Furthermore, in the fuel cell system according to the invention, it is possible to dispense, inter alia, with additional reformate coolers between gas-cleaning stage and fuel cell and/or between gas-generation device and gas-cleaning stage, since the cooling medium is substantially at the operating temperature of the fuel cell. As a result, a plurality of functions are combined in a single component, so that fewer feed lines and discharge lines are required. Moreover, it is also possible to reduce the weight and space required, with an associated fall in costs.
The variation in coolant flow with the aid of a bypass line with associated bypass valve and control unit makes it possible to establish a temperature distribution in the reactor bed which is as constant as possible in terms of its dynamics, and thus to achieve a minimal CO output concentration under any load.